<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .wrap {
            display: flex;
            width: 600px;
            height: 200px;
            border: 1px solid black;
        }
        .child1 {
           flex-basis: auto;
           /* width: 100px; */
            margin-left: 100px;
            /* flex-grow: 1; */
            background-color: orangered;
        }
        .child2 {
            width: 200px;
            background-color: yellowgreen;
        }
        .child3 {
            width: 200px;
            background-color: skyblue;
        }
    </style>
</head>
<body>
    <div class="wrap">
        <div class="child1">66</div>
        <div class="child2"></div>
        <div class="child3"></div>
    </div>
</body>
</html>